Processes for dewaxing petroleum distillates are well known. Dewaxing is required when highly paraffin oils are to be used in products which must be mobile at low temperature. The high molecular weight straight chain normal and branched parrfains present in such oil are waxes which cause high pour points and high cloud points in the oils. If adequately low pour points are to be obtained, the waxes must be wholly or partially removed.
A number of dewaxing processes is known in the petroleum refining industry. The catalyst which have been proposed for the dewaxing processes have usually been zeolite which have a pore size which admits the straight chain, waxy n-parrafins but which exclude more highly branched chain materials and cycloadditions. Intermediate pore size zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and the synthetic ferrierites have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. No. 3,700,585 (Re 28398); U.S. Pat. Nos. 3,894,938; 3,933,974; 4,176,050; 4,181,598; 4,222,855; 4,259,170; 4,229,282; 4,252,499; and 4.247,388.
A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. The Mobil Lube Dewaxing Process (MLDW) is also described in Chen et al “Industrial Application of Shape Selective Catalysis” Catal. Rev.-Sci. Eng. 28 (283), 185-264 (1986) especially pp. 241-247, to which reference is made for a further description of process. Reference is made to these disclosures for a description of various catalytic dewaxing processes.
In catalytic dewaxing processes of this kind, the catalyst becomes progressively deactivated as the dewaxing cycle progresses. To compensate this, the temperature of the dewaxing reactor is progressively raised in order to meet the target pour point of the products. There is a limit, however, to which the temperature can be raised before the properties of the product, especially oxidation stability become unacceptable. For this reason, the catalytic dewaxing process is usually operated in cycles with the temperature being raised in the course of the cycle from a low start of cycle (SOC) value, typically about 260° C., to a final, end of cycle (EOC) value, typically about 360°, after which the catalyst is reactivated or regenerated for a new cycle.
The use of a metal hydrogenation component on the dewaxing catalyst has been described as a highly desirable expedient, both for obtaining extended dewaxing cycle durations and for improving the reactivation. U.S. Pat. No. 4,683,052 discloses the use of noble metals component e.g., Pt, Pd as a superior metals such as nickel for this purpose. During the dewaxing cycle itself, nickel on catalyst was thought to reduce the extent of coke lay down by promoting transfer of hydrogen to coke precursors formed on the catalyst during dewaxing reactions. Similarly, the metal was also thought to promote removal of coke and coke precursors during hydrogen reactivation by promoting hydrogen transfer to these species to form materials which could be more readily desorbed from the catalyst.
European Patent Application No. 225,053 discloses a process for producing lubricant oils by partially dewaxing a lubricant base stock by isomerization dewaxing step carried out using a large pore high silica zeolite dewaxing catalyst such as silica Y or zeolite beta which isomerizes the waxy components of the base stock to less waxy branched chain isoparaffins. The selective dewaxing step may be either a solvent, e.g., MEK dewaxing operation or a catalytic dewaxing, preferably using a highly shape zeolite such as ZSM-22 or ZSM-23.